At this point we are working on project for about three months., and will be posting past reports. And first one starts like this:

Do not buy MG811 module from Sainsmart! We recommend one from DFRobot.
The catch is, MG811 needs 6V. No it's not a typo, it's six (6) volts. Sainsmart didn't get it, DFRobot did, and included a step-up on their module.

Also, we gave up DSM501A dust sensor by ITead. We don't know whats wrong, but didn't get any meaningful readings. (pointers appreciated). Instead we bought Grove dust sensor from Seeed, seems fine.

We acquired other gas sensors from Sainsmart, and they seem fine.